The weapon itself is described as "a long metal tube" which is pointed at the victim.

"All was ready, yet before emerging onto the upper Eartth, they decided to have a sure, deadly weapon against the races of man, who might molest them in their work. So they developed a deadly ray, a ray that crumbles a human body into a puff of white powder instantly..."

...As a number of them [Martians] began to climb up the pillar, the blue ray of death flashed out from inside the box and cut a wide swath through their numbers, reducing those it touched to white powder in an instant!"

Fans of Star Trek TOS may recall the episode By Any Other Name; an alien ray reduced crew members to polyhedra of crumbling salts. The good news was that the person could be reconstituted - the bad news was once a block was crumbled, that person was gone.